1. Field of the invention
The present invention concerns a transformer with a closed ferrite magnetic circuit including an air gap, in particular a transformer of this type used in switching power supplies.
2. Description of the Prior Art switching power supplies operating in accumulation or flyback mode require energy storage in the ferrite magnetic circuit of a transformer before this energy is transferred to the electric circuit connected to the transformer. This energy storage requires the use of a magnetic circuit with an air gap. Conventionally, a type of transformer for switching power supply is constituted of a magnetic circuit composed of two half-circuits of ferrite having a section in the form of an "E" and winding support carrying one or more electrically separated coils. The winding support is provided with an axial hole in which are inserted the central arms of the "E"s. The magnetic circuit and the winding support are assembled by glueing. One of the ferrite half-circuits is preferably stuck to the winding frame, then the two magnetic half-circuit are joined by sticking together the sides facing the outer arms of the circuits. This leaves an air gap between the inner arms. Such an assembly guarantees good mechanical strength of the magnetic circuit and the coil. The air gap easily absorbs mechanical stresses, provoked in particular by thermal shocks, in conformity with present norms. However, when the gap is an air gap, it generates magnetostrictive phenomena which induce audible humming in the transformer. Magnetostriction is a type of elastic deformation related to magnetism, i.e. the geometric characteristics of the ferrite vary as a function of the magnetic induction.
In order to overcome this difficulty, we proposed in the French patent application no. 87 15202, filed on 3rd Nov. 1987, in the name of the Applicant, to place in the gap between the inner arms a material of the silicone family. Although this material has given satisfactory results for fixed frequencies, this not the case when the transformer has to operate at variable frequencies and the present invention overcomes this deficiency. In addition, the silicones which must be used are neutral silicones, which have the disadvantage of being only poorly adherent.